Always take a ref for all event so that the object is only freed when

the op is released.

Reset sal pointer held by all ops when destroying the sal object Avoid attempting to send messages if the sal has already been destroyed

Improve documentation of conference participant list type core and conference parameter attributes as it is only applicable to servers

Rework linphone_core_get_call_history in such a way that the call log list held by the core is updated to match exacty what is stored in the database and its members are reused as much as possible. Basically for every call log stored in the database, the method will look if one is already in the list and reuse it.

Fix test to account for call log server incoming messages and internal event class changes